The line structure of a raster scanned television display becomes more visible as the size and brightness of the display increase. This undesirable effect may be avoided by doubling the number of scanning lines in each field and displaying the lines in progressive scan (non-interlaced) form. An example of a progressive scan receiver, wherein the added lines are replicas (repeats) of the original scan lines, is described by R. A. Dischert in U.S. Pat. No. 4,415,931 entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES. In another form of progressive scan receiver, the "extra" lines for display are obtained by interpolation of adjacent vertical lines of the incoming video signal. See, for example, U.S. Pat. No. 4,400,719 of K. H. Powers entitled TELEVISION DISPLAY WITH REDUCED LINE SCAN ARTIFACTS.
The receivers of Powers and Dischert do not require any change in existing television broadcast standards. It has been recognized, however, that such receiver systems are subject to certain motion dependent artifacts characteristic of the interlaced scanning format employed in the cameras of conventional broadcast systems. Examples include effects such as serration of the edges of horizontally moving objects, double images, aliasing, and so on. Various arrangements have been proposed which avoid interlace motion artifacts by using progressive scanning (or spot wobble scanning) in both the camera and in the receiver display. The difficultly is to encode the progressive scan camera signal in such a manner as to be compatible with existing television transmission channels and standard (interlace scan) receivers as illustrated by the following examples.
As a first example, Oakley et al. in U.S. Pat. No. 4,429,327 entitled COMPATIBLE TELEVISION SYSTEM WITH INCREASED VERTICAL RESOLUTION describe a compatible NTSC system in which a spot wobbling technique is used in a camera to generate two lines of video for each line scanned. The lines are added to produce a compatible luminance signal and subtracted to provide a difference signal representing vertical resolution. Provision is made for transmitting the difference signal along with the sum signal by frequency interleaving the difference signal with the chroma signal in locations representing high frequency vertical information. Alternatively, Oakley et al. suggest transmitting the difference signal via a separate transmission channel. The sum and difference signals are recombined in a receiver for display on a kinescope employing spot wobble synchronized with that of the camera.
As a second example, K. H. Powers in U.S. Pat. No. 4,533,951 entitled SYSTEM FOR GENERATING AND DISPLAYING A COMPATIBLE HIGH DEFINITION TELEVISION SIGNAL BY PROGRESSIVE SCANNING, describes a spot wobble system in which a high definition (spot wobble) camera signal is low pass filtered to provide a limited bandwidth signal compatible with standard broadcast receivers. A difference signal is produced by subtracting the high definition and limited definition signals. In a receiver, the limited definition and difference signals are added to recreate the high definition signal for display. The difference signal and its addresses are transmitted to the receiver during the vertical blanking interval of the limited definition signal. In order to reduce the data rate of the difference signal, its generation is inhibited in regions of the picture which exhibit frame-to-frame motion.
As a further example, Adelsen et al. in U.S. Pat. No. 4,523,220 entitled COMPATIBLE HIGH DEFINITION TELEVISION SYSTEM UTILIZING HADAMARD BASIS FUNCTIONS describe a system in which a video signal provided by a high definition camera is converted into one sum signal and three difference signals. The sum signal represents the average of four contiguous pixels of the high definition signal and is compatible with standard television receivers. The three difference signal represent vertical, diagonal and horizontal resolution and one obtained by subtracting various combinations of the four pixel group that provides the compatible sum signal. Adelsen et al. suggest that the difference signals may be transmitted in the vertical or horizontal blanking intervals of the sum signal for subsequent reconstitution of the signal high definition signal in a receiver.